1. Field of the Invention
The present invention pertains to the use of block copolymers containing a reactive monomer or monomers in two or more blocks via controlled free radical polymerization and use of the composition of matter as additives for the preparation of silicate-polymer composites.
2. Description of the Related Art
In the parent invention, U.S. patent application Ser. No. 11/508,407, a block copolymer was discovered that performs well as a compatibilizer. In one embodiment, a process was described for making a block copolymer having a first block with functional groups provided via an acrylic monomer, where no purification step was used after polymerizing the first block so that an amount of unreacted residual monomer, which has functional groups, was intentionally left in the reaction product from the first step. A second block was added to the first block to form the block copolymer. The second block was preferably polymerized from at least one vinyl monomer and the residual unreacted monomer that has functional groups. Functional groups were consequently added into the second block, as well as into the first block, which was discovered to provide a block copolymer that has a good performance as a compatibilizer.
A typical blend composition in the parent comprises from about 1 to about 98 wt % of a first thermoplastic polymer, which has functional groups selected from the group consisting of amino, amide, imide, carboxyl, carbonyl, carbonate ester, anhydride, epoxy, sulfo, sulfonyl, sulfinyl, sulfhydryl, cyano and hydroxyl, from about 0.01 to about 25 wt % of a block copolymer that includes a first block, which has monomeric units of a functionalized acrylic monomer and monomeric units of a vinyl monomer and a second block, which has monomeric units of one or more vinyl monomers and monomeric units of the functionalized acrylic monomer in the first block, and from about 1 to about 98 wt % of a second thermoplastic polymer, which is miscible with or compatible with the second block of the block copolymer, and where the acrylic monomer has functional groups that should react with the functional groups in the first thermoplastic polymer.
The parent invention provides in one embodiment a process for making a block copolymer, which includes the steps of reacting an acrylic monomer, which has functional groups, and one or more vinyl monomers in the presence of a free radical initiator and a stable free radical to form a reaction product, where the reaction product includes residual unreacted acrylic monomer, and reacting one or more vinyl monomers with the reaction product to form a second block, where the second block incorporates the residual unreacted acrylic monomer.
The present invention concerns an application where the parent invention is used in the preparation of silicate-polymer composites. Clays and other fillers are added to polymers to provide a composition that is desirable in one or more aspects.
Silicate-polymer nanocomposites offer a number of significant advantages over traditional silicate-polymer composites. Conventional silicate-polymer composites usually incorporate a high content of the inorganic fillers—from 10 to as much as 50 weight percent (wt. %)—to achieve desired mechanical or thermal properties. Polymer nanocomposites can reach the desired properties, such as increased tensile strength, improved heat deflection temperature and flame retardance, with typically 3-5 wt. % of the nanofiller, producing materials with specific gravity close to that of the unfilled polymer, good surface appearance and better processability than traditional reinforcements. Other properties of nanocomposites such as optical clarity and improved barrier properties cannot be duplicated by conventionally-filled resins at any loading. (Bins & Associates, Plastics Additives & Compounding, 2002, 30-33.)
One general approach to prepare polymer nanocomposites is to employ an approach known as intercalation chemistry of layered inorganic solids. In this approach polymer chains can be inserted into the interlayer space of these layered solids. The layered solids include graphite, clay minerals, transition metal dichalcogenides, metal phosphates, phosphonates and layered double hydroxides, etc. Among them, clay minerals have been widely used and proved to be very effective due to their unique structure and properties. Such minerals include natural clays of the smectite family (e.g., montmorillonite, hectorite and saponite) and synthetic clays (fluorohectorite, laponite y magadite). Among them, montmorillonite and hectorite are to date the most widely used ones. (Zeng, Q.; Yu, A.; Lu, G.; Paul, D., J. Nanosci. Nanotech. 2005, Vol. 5, No. 10, 1574-1592.)
Dispersion of layered clays into a polymer matrix can lead to either a conventional composite or a nanocomposite depending on the nature of the components and processing conditions. Conventional composites are obtained if the polymer can not intercalate into the galleries of clay minerals. The properties of such composites are the similar to that of polymer composites reinforced by microparticles. (Zeng, Q.; Yu, A.; Lu, G.; Paul, D., J. Nanosci. Nanotech. 2005, Vol. 5, No. 10, 1574-1592.). On the other hand, if the polymer intercalates into the clay galleries two extreme nanostructures can result. One is an intercalated nanocomposite, whose ordered layers are maintained with the polymer existing between the silicate layers, in addition to surrounding the clay particles. The other is an exfoliated or delaminated nanocomposite, in which the silicate layers are completely dispersed within a continuous polymer matrix, and thus the silicate (clay) particles lose the ordered structure. In general, exfoliated nanocomposites exhibit greater improvements to the material properties than exfoliated nanocomposites, and therefore is typically the more desired scenario. (Argoti, S. D.; Reeder, S.; Zhao, H.; Shipp, D. A. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2002, 43, 267-268.) The complete dispersion of clay platelets (silicate layers) in a polymer optimizes the number of available reinforcing elements for carrying an applied load and deflecting cracks. The coupling between the tremendous surface area of the clay platelets ˜760 m2/g. and the polymer matrix facilitates stress transfer to the reinforcement phase, allowing for tensile and toughening improvements. Conventional polymer—clay composites containing aggregated nanolayer tactoids ordinarily improve rigidity, but they often sacrifice strength, elongation and toughness. However, exfoliated clay nanocomposites, have shown improvements in all aspects of their mechanical performance. High aspect ratio nanolayers also provide properties that are not possible for larger-scaled composites. The impermeable clay layers mandate a tortuous pathway for a permeant to transverse the nanocomposites. The enhanced barrier characteristics, chemical resistance, reduced solvent uptake and flame retardance of clay-polymer nanocomposites all benefit from the hindered diffusion pathways through the nanocomposite. (LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Appl. Clay Sci. 1999, 15, 11-29).
Considering the importance of obtaining exfoliated clay nanocomposites in polymeric matrices, several processing strategies have been proposed, which are described below. (Zeng, Q.; Yu, A.; Lu, G.; Paul, D., J. Nanosci. Nanotech. 2005, Vol. 5, No. 10, 1574-1592.)
1. In situ polymerization. In this technique, monomers are intercalated into layered clays and subsequently polymerized within the gallery via heat, radiation, pre-intercalated initiators or catalysts. This strategy has been applied mainly for condensation polymers such as polyurethanes, polyamides, polyethylene terephthalate, epoxy, polylactones and polysiloxanes, polyethylene oxide, although it has also been applied for other polymers like polystyrene. (LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Appl. Clay Sci. 1999, 15, 11-29.) There are several patents in which nylon nanocomposites are formed using a monomer that also acts as a swelling agent or tensoactive, since it has a head group formed by an ammonium, pyridinium, sulfonium or phosphonium group. (U.S. Pat. No. 4,739,007, issued to Usuki et al.; U.S. Pat. No. 4,810,734, issued to Kawasumi, et al.; and U.S. Pat. No. 4,889,885, issued to Usuki et al.) Some of the disadvantages of this technique are: i) Clay exfoliation depends on the extent of clay swelling and diffusion rate of monomers in the gallery and ii) oligomers may be formed upon incomplete polymerization. Rodak et al. in U.S. Patent Application Pub. No. 20060211803 modify clay by contacting it with an unsaturated cationic compound and an alkoxyamine or an adduct thereof. The resulting pre-activated clay, which contains a cationic alkoxyamine bound to the clay, may be further treated with a monomer to provide a polymer that is bound to the clay, thereby forming a nanocomposites material. The strategy is complicated since it requires the use of a sophisticated cation which bears a double bond capable of reacting with an alkoxyamine. The reaction between the alkoxyamine and the cation is made in a non-water Solution. Further polymerization with the monomer that will form the polymeric matrix is limited to monomers polymerizable by a controlled radical polymerization process.
2. Solution Exfoliation. In this case layered clays are exfoliated into single platelets using a solvent in which the polymer is soluble. The polymer is then mixed with the clay suspension and adsorbed onto the platelets. The solvent is finally eliminated from the clay-polymer complex through evaporation. This technique is usually employed to modify polar polymers such as epoxy, polyimide, polyethylene, poly(methylmethacrylate) and also polymers made by emulsion processes such as styrene-butadiene and styrene-acrylonitrile copolymers. Some of the disadvantages of this technique are: i) a compatible polymer-clay solvent system is not always available, ii) use of large quantities of solvent and iii) co-intercalation may occur for solvent and polymer.
3. Melt intercalation. In this case layered clays are directly mixed with the polymer matrix in the molten state. The formation of polymer nanocomposites is driven by different forces depending on the technique used. This is a usual technique for styrene and polyolefins. Although in the case of polyolefins a compatibilizer is required. The main disadvantage of this technique is the slow penetration (transport) of polymer within the confined gallery. Comparing this strategy with the first two, it has an environmentally benign approach since no solvent is required and in this case nanocomposites can be processed with conventional plastic extrusion and molding technology. Some of the patents applying this strategy can only achieve intercalation of the polymer (polystyrene or poly(ethylene oxide)) in the clay galleries, but a complete exfoliation is not achieved. (U.S. Pat. No. 5,955,535, issued to Vaia et al.)
Clays consist of stacked aluminosilicate layers that can be separated, but the clay layers, which are held together by electrostatic forces, cannot be broken into separate layers by simple shear, and for that reason, organic modification of the clay is necessary to achieve separation of the stacked clay layers. To obtain a larger spacing, many studies on nanocomposite formation have focused on the modification of clay by introducing organic molecules into the clay layers through a cation-exchange reaction (typically Na+ or K+, are exchanged for organic cations). Hence, there have been many attempts at the organic modification of clay either using organic cations, such as ammonium, imidazolium, phosphonium, stibonium, tropylium, etc., or introducing different organic groups onto these cations. The objective of the modification of the clay is to provide hydrophobic characteristics to the hydrophilic surface of a clay layer, which may permit the entry of organic polymers; at the same time, the spacing of the clay is increased. (Nam, J. B.; Wang, D.; Wilkie, C. A. Macromolecules 2005, 38, 6533-6543.) In some cases, the alkyl ammonium cation can also act as an initiator for in situ polymerization. There are several types of tensoactives (organic cations) that can be selected according to the specific application although in most of the cases the substituents are chains derived from tallow, coconut oil that can or cannot be hydrogenated. (Nam, J. B.; Wang, D.; Wilkie, C. A. Macromolecules 2005, 38, 6533-6543; U.S. Pat. No. 5,747,560, issued to Christiani et al.; U.S. Pat. No. 5,663,111, issued to Gadberry et al.; U.S. Patent Application Pub. No. 2002/0037953 filed by Lan et al.) Organoclays are commercially available materials from producers such as: Southern Clay Products Inc. of Gonzales, Tex. (http://www.nanoclay.com/) under the trade name of Cloisite®, Süd-Chemie Inc. of Munich, Germany (http://www.sud-chemie.com) under the trade name of Nanofil® and Nanocor of Arlington Heights, Ill., a subsidiary of AMCOL International Corporation. (http://www.nanocor.com) under the trade name of Nanomer®.
Among the disadvantages of commercially-available organoclays are: i) the limited amount of organic cations do not guarantee a good interaction between the polymer and the clay, and a good exfoliation is not easy to achieve; and ii) the low thermal stability caused by the thermal degradation of amines according to the Hofmann mechanism. (J. March, Advanced Organic Chemistry, McGraw-Hill, 7th ed.) To overcome these problems a number of strategies have been explored.
In U.S. Pat. No. 6,828,367, Campbell explored the use of an alkyl amine and an aromatic diamine, which has higher thermal stability and can be further reacted. This solution is limited to polymer or polymeric precursors capable of reacting with amine groups, and the patent only discloses improvement in mechanical properties, but no characterization is provided to demonstrate a complete exfoliation. Campbell mixes an inorganic cation such as (Ti(OC3H7)4, Zr(OC3H7)4, PO(OCH3)3, PO(OC2H3)3, B(OCH3)3, B(OC2H5)3 with an organic intercalant (a water soluble polymer like polyvinyl alcohol, polyclicol, PVP, polyacrilic acid, etc). The organic agent is further calcinated before mixing the modified clay with the thermoplastic or thermoset to be modified, or the organic modifier can have organic groups that interact with the polymeric matrix through some kind of chemical or electrostatic interaction. (International Patent Application Pub. No. WO9731057 for inventors Nichols and Chou.) A variation of this strategy is disclosed in U.S. Pat. No. 5,552,469, issued to Beall et al., which describes the preparation of intercalates derived from certain clays and water-soluble polymers such as polyvinyl pyrrolidone, polyvinyl alcohol and polyacrylic acid. Although the specification describes a wide range of thermoplastic resins including polyesters and rubbers that can be used in blends with these intercalates, there are no examples teaching how to make such blends and if the intercalates transform to exfoliated materials when mixed with the claimed polymers. Another disadvantage is that these strategies might only be adequate for a small group of polymers or polymeric precursors which are compatible with the organic intercalants.
Whereas most of the patents related to clay modification are related to discrete organic molecules bearing a positive charge, fewer examples describe the use of oligomeric or polymeric species to intercalate or exfoliate clays. The use of oligomeric or polymeric species tends to enhance the interaction between the polymer and the clay, since the tensoactive species is chosen to be compatible with or of similar composition as the polymeric matrix.
The use of poly(oxypropylene)diamine to intercalate and exfoliate clays is one example. The amine group contained in the poly(oxypropylene) diamine can be further reacted with the polymeric matrix. (Chu, C.-C.; Chiang, M.-L.; Tsai, C.-M.; Lin, J.-J. Macromolecules 2005, 38, 6240-6243.) This solution is adequate for polymers or polymeric precursors capable of reacting with amine groups, although the document does not include examples of polymers modified with this type of modified clays. A variation of this strategy contemplates the modification of polycaprolactones and polyesters by reacting them with diamines. (U.S. Pat. No. 6,384,121, issued to Barbee et al.; and U.S. Patent Application Pub. No. 2002/0137834 filed by Barbee et al.) The resulting resins are protonated in water and used to modify clays (typically sodium or organically modified montmorillonite) and intercalates are obtained. A variation of this strategy is mixing amorphous oligomers (typically polyamides) with organoclays. (U.S. Patent Application Pub. No. 20020119266 filed by Bagrodia et al.) The resulting organoclay is then added to polymers (polyesters and polyamides) in the molten state and materials with improved mechanical, optical or oxygen permeability reduction are claimed to be obtained. This strategy is also very specific for polymers that can react with an amine.
Recent publications refer to the use of block copolymers as organic intercalants for clays. U.S. Pat. No. 6,579,927, issued to Fischer, describes the use of a block copolymer or graft copolymer comprising structural units (A), which are compatible with the clay, and one or more second structural units (B), which are compatible with the polymeric matrix. Although the composition of the structural units (A) and (B) are described, there appears to be no description or example of how to prepare these block or graft copolymers, and the performance of the modified clays in several polymers is described vaguely, making it unclear if a complete exfoliation was achieved or not.
Muhlebach et al. disclose in U.S. Patent Application Pub. No. 20060160940 a process for manufacturing nanoparticles by intercalating and/or exfoliating natural or synthetic clays using block or comb copolymers having one cationic block and at least one nonpolar block, which are prepared by CRP. The block copolymer has a cationic block A, wherein the cation is based on at least one nitrogen atom, and a nonionic block B, both blocks having a polydispersity between 1 and 3, or a comb copolymer having a cationic backbone A, wherein the cation is based on a nitrogen atom and nonionic oligomeric/polymeric chain B attached thereto, the cationic backbone A having a polydispersity between 1 and 3 and the nonionic side chains having a polydispersity of 1.0-1.8. In order to obtain one or more neutral or nonionic blocks, the process for preparing diblocks requires the isolation of the first block before adding the monomers that will constitute the second block. The process for intercalating the clay requires the use of special additives like Dowanol® (1-methoxy-2-propanol) and a long period of stirring and heating (24 h, 60° C.). The intercalated clay purifying process requires a final washing with ethanol before drying. Finally, the detailed description mentions the use of the nanocomposites dispersions in several applications, but there appears to be no detailed description of how to use them, examples or claims related to the application of these intercalates in the modification of polymeric matrices.
The development of new intercalating agents to improve thermal stability and miscibility of the clay with the polymeric matrix in order to obtain exfoliated clays for polymer reinforcement is an area of intense research, and in most of the cases a mixture of intercalated and exfoliated clay is found. Even for nylon/nanoclay composites, which show a large amount of exfoliated clay and an outstanding mechanical performance when prepared by in situ polymerization, there is a considerable amount of research focused on developing a clay intercalant that can modify nylon using a melt intercalation process. Melt intercalation allows compounders to directly incorporate the clay to a commercially-available polymer using conventional plastic extrusion and molding technology, which offers advantages compared to the in situ polymerization process which can only be done commercially by polymer producers, since a polymerization process is involved.
In the case of polymers with very low polarity such as polyolefins, the panorama is more complicated, since organic clays are not intercalated at all when they are added directly to polymers like polyethylene or polypropylene. (Kim, Y.; L. White, J. Journal of Applied Polymer Science, 2003, 90, 1581-1588.) To overcome this problem, compatibilizers such as maleic anhydride grafted polypropylene (PP-g-MA) have been used. (Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.; Okada, A. Macromolecules 1997, 30, 6333-6338.) When PP-g-MA is used in a 3:1 ratio with the organoclay, good intercalations are obtained. (Makoto, K.; Arimitsu, U.; Akane, O. Journal of Applied Polymer Science, 1997, 66, 1781-1785.) The amount of PP-g-MA in the final blend may vary but it's usually around 20-30% wt considering the amount of PP as 100%. This large amount of PP-g-MA has some disadvantages. (Lee, E. C.; Mielewski, D. F.; Baird, R. J. Polymer Engineering and Science 2004, 44, 1773-1782.) First, since the molecular weight of PP-g-MA is usually low, it causes a detriment in the mechanical properties of the nanocomposites. Second, PP-g-MA has a higher cost than PP, which adds to the total cost of the nanocomposites.
Polypropylene's attractive combination of low cost, low weight, heat distortion temperature above 100° C., and extraordinary versatility in terms of properties, applications, and recycling have stimulated exceptional growth of polypropylene production. There is a considerable interest in obtaining PP/clay nanocomposites, and most of it is focused on a better processing methodology to exfoliate intercalated clays using a compatibilizer or finding a better compatibilizer.
Styrene maleic anhydride copolymers have been evaluated as compatibilizers of organic clays and polypropylene. (Fang-Chyou, C.; Sun-Mou, L.; Jong-Wu, C.; Pei-Hsien, C. Journal of Polymer Science: Part B: Polymer Physics 2004, 42, 4139-4150.) The results compared with PP-g-MA depend on the type of organoclay and on the amount of maleic anhydride container in the copolymer.
PP grafted with a copolymer of maleic anhydride, methyl methacrylate and butyl acrylate has also been studied (Ding, C.; Jia, D.; He, H.; Guo, B.; Hong, H. Polymer Testing 2005, 24, 94-100). This strategy increases the molecular weight of grafted PP improving the final mechanical properties. The amount of grafted PP can be reduced to 2% wt relative to the amount of PP. Clay intercalation is improved by the addition of grafted polypropylene but it is not exfoliated. Another disadvantage of this strategy is that the grafted PP is not commercially available. A variation of this strategy is the use of PP grafted with a copolymer of styrene and glycidyl methacrylate and with acrylic acid (M. L. López-Quintanilla; S. Sánchez-Valdés; L. F. Ramos de Valle; Medellín-Rodríguez, F. J. Journal of Applied Polymer Science 2006, 100, 4748-4756). The degree of intercalation depends on the amount of grafted polypropylene and on the type of grafted polypropylene. In general, the results resemble those obtained with PP-g-MA.
Hydroxylated PP has also been tested as an additive to improve organoclay incorporation, obtaining improved intercalations, but the results obtained do not improve the performance of PP-g-MA. (Makoto, K.; Arimitsu, U.; Akane, O. Journal of Applied Polymer Science, 1997, 66, 1781-1785.)
Other strategies include the use of PP-g-MA and thermoplastic polyolefins (polybutadiene, EPDM, ethylene-octene copolymers) in order to modify polyolefins. (International Patent Application Pub. No. WO 2005056644 for inventors Jarus and Cicerchi.) Using this strategy, the amount of PP-g-MA can be reduced to around 5% wt in the final composition, and the thermoplastic polyolefin is added. The examples only compare performance of PP/thermoplastic polyolefins blends against the same blends with PP-g-MA and nanoclays, so there is no evidence of the better performance compared with only using PP-g-MA. Since the thermoplastic polyolefins also modify the mechanical properties and X-ray diffraction information is not provided, it is not clear whether the thermoplastic polyolefin improves intercalation or exfoliation or if it is acting as an impact modifier. Another strategy is the treatment of the organoclay with silanes in order to improve dispersion in PP-g-MA matrix (U.S. Pat. No. 6,632,868, issued to Quian et al.) In this case mechanical tests show performance improvement, but since X-ray diffraction or TEM characterization is not disclosed, the amount or degree of intercalation or exfoliation is not revealed.
As is evident in the discussion above, a great deal of work has been done in the field of producing clay or silicate and polymer composites. While significant improvements have been made over the years in improving the compatibilization of clays with polymers, there is still considerable room for improvement.